Modem requirements for power supplies generally specify operating efficiencies significantly above requirements acceptable in the past. This is due in part to the stringent environments in which modem power supplies must operate. These environments require higher power supply power densities and also limit the amount of dissipated heat that is allowable as compared with past requirements. Accordingly the individual components as well as the overall power supply must be operated in the most efficient manner possible.
A major source of power loss in switching type power supplies is to be found in the power dissipation occurring in individual components of the power supply. One such component, which is a major source of power dissipation, is the semiconductor power switch. Power dissipation is caused by the simultaneous occurrence of voltage across and current passing through the switch during its turn-off and turn-on transitions. During the turn-on transition, for example, the time interval of reduction of voltage across the switch is concurrent with a current flow through the switch and power dissipation occurs. This power dissipation is a direct function of the current level passing through the power switch. With the recent trend to higher output currents required of power supplies this has become a serious problem to the power supply designer.
One approach to reducing this turn-on transition dissipation has been the use of snubber circuits to control the voltage and current of the power switch during switching transitions. These snubber circuits have been used very successfully in high output power supplies of the past. The characteristic of the conventional snubber circuit involves a voltage reduction time interval that is dependent on the amplitude of current flowing through the snubber circuit. The design of the snubber circuit requires the selection of a reduction time interval that can handle worst case scenarios i.e. anticipated worst case conditions of line and load. In the instance of switching power supplies used for power factor enhancement in coupling an AC line to a circuit, the constantly changing instantaneous currents combined with the reduction time interval induces unnecessary conduction looses in the snubber circuit itself.